While fixed antennas are typically used as base station antennas in mobile communication systems, vertical variable down-tilting antennas are also used due to a reduction in labor and optimization of coverage benefits. A vertical variable down-tilting antenna can adjust phase at a vertical array by use of a phase shifter, thereby controlling an antenna beam vertically in accordance with the coverage of a cell site.
In recent years, a technique for horizontally steering antenna beams in the directions of sectors according to the distribution of subscribers within a cell site has been developed. There are two means of providing horizontal control of antenna beams: electrical horizontal beam control through electrical phase control of a signal provided to each column, and mechanical beam control that utilizes horizontal steering using a one-column antenna.
The mechanical beam control is preferred since its size and has the electrical advantage of not causing horizontal side lobe. The vertical beam control is performed by a separate operation and thus it is applicable to both vertical tilting and horizontal steering.
The use of an antenna equipped with the two-dimensional control functions of vertical tilting and horizontal steering creates dynamic network optimization according to a wireless network's capacity and coverage requirements. However, problems may occur in actual cell sites when only two-dimensional beam antennas are utilized. A typical sector configuration, i.e., a three 120-degree sector configuration, when horizontal steering direction is adjusted according to subscriber distribution, shadowing can be produced or an overlapped zone increase between sectors.
Accordingly, for adjustment of the horizontal steering direction, altering the horizontal beam width is required to suppress the shadowing and minimize the overlap zone.
It has been difficult, though, to provide an easy and low-cost method of altering the horizontal beam width. Typically, there are three methods available to change the horizontal beam width.
The first consists of adjusting the angle and length of a reflector in a one-column antenna. It is a classic method used for a vertical polarization antenna.
An example of the first method is disclosed in “Ref. Mobile Antenna System Handbook, K. Fujimoto and J. R James pp. 133-134”. However, distinctive drawbacks of the first horizontal beam width changing method are that an antenna must be very large due to a valid reflector length, and the isolation and cross polarization of a dual polarization antenna widely used at preset are degraded.
The second method to change horizontal beam width is a typical antenna technique in which a three or more-column antenna is horizontally implemented in order to change the antenna beam width through control of the distribution ratio and phase of each column. An example of the second method found in a Korean Patent Application No. 2003-7000418 entitled “Cellular Antenna”, filed by “Andrew Corporation”. This second method is not viable for commercialization in a mobile communication base station.
While a predetermined beam width is realized with the use of a one-column or two-column antenna in a typical mobile communication base station, the second method requires at least a three-column antenna. Therefore, antenna size and cost are increased. Moreover, to change the distribution ratio and phase, expensive and high-loss parts are used, thereby decreasing antenna gain. Therefore, an antenna employing this method is used for military purposes.
The third method utilizes a two- or more-column antenna which is implemented horizontally, and the horizontal steering directions of the reflectors in the columns are controlled to cross each other mechanically, thereby controlling the beam width. In practice, it is hard to form a typical antenna beam suitable for a sector with this kind of antenna. An example of the third method is found in a Korean Patent Application No. 2003-95761 entitled “Apparatus for Controlling Antenna Beam in a Mobile Communication Base Station”, filed by the present applicant. When a wide beam width is obtained by changing the antenna beam width, ripples are created in the forward direction of the antenna and a radiation pattern other than “Sharp Roll-off” increases an overlap zone between sectors. This third method also requires at least a two-column antenna.